1. Field of the Invention
The present invention relates to an electrification apparatus and an image forming apparatus such as a copier, facsimile, or printer which uses the electrification apparatus.
2. Description of the Prior Art
In order to form an image by electro-photography, a so-called electrification needs to be performed which positive or negative electric charge is previously applied to a photosensitive body to maintain charge carrier toner.
Conventionally, to implement this electrification, a corona electrification method that uses fine metal wires for corona discharge has been used. This conventional corona electrification method, however, has a problem that ozone generates due to the discharge.
In recent, a contact electrification method is adopted in which a photosensitive body is contacted with toner to be electrified. This contact electrification method uses a charger having a type such as a rotated roller or non-rotating brush, and uses two kinds of electrification methods, that is, an electric charge injection method and a micro gap discharge method, each of which has its own drawbacks and advantages.
These contact electrification methods feature that they perform a discharge or an electric charge injection with a very short distance and thus generate very little ozone during discharge. These methods have, however, the maximum disadvantage due to the use of contact in that, when rotating toner adheres to a photosensitive body to come to an electrification unit and cannot be completely removed, then this toner adheres to the electrification unit and gradually accumulates to deteriorate electrification power or retransfer to the photosensitive body, causing imperfect image.
In order to avoid such an undesirable situation, non-contact type-electrification method is desirable and, in order to minimize the ozone generation, a gap between a photosensitive body and a charger must be reduced.
Nevertheless, in the corona discharge method using fine metal wires, the vibration of the fine metal wires occurs, as can be seen from a careful observation of the discharge by this method.
In a non-contact type-electrification method, its electrification principles are also based on the transfer of corona ions although it uses micro gap. Calculating based on Paschen""s law, the method""s minimum distance at which discharge starts in an atmospheric pressure is about 70 xcexcm.
It is difficult, however, for the vibrating wires to assure this distance of 70 xcexcm throughout the full width of wires, and the center portion of the wire may contact with a photosensitive body. Moreover, such a contact portion with the photosensitive body may cause itself to have short circuiting to damage the photosensitive body, making it impossible to provide uniform electrification on the entire photosensitive body.
There is an attempt in a conventional roller method where end parts of a roller and the like have predetermined thickness to keep such a distance. This attempt has, however, a problem in that when a roller always contacts a photosensitive body to slide with the body, the photosensitive body or the roller oscillating part begins to abrade away, resulting in the loss of an uniform electrification due to the repeated use.
In view of the above, it is an object of the present invention to provide an electrification apparatus and image forming apparatus that reduce ozone generation, provide uniform electrification of a photosensitive body, and have high durability.
In order to achieve the above object, the present invention provides an electrification apparatus for providing electrification to a photosensitive body in electro-photography. The electrification apparatus comprises a first magnet means composed of a magnetized base body obtained by magnetizing a base body of the photosensitive body or a magnet configured inside of the base body of the photosensitive body; a second magnet means magnetically levitated by the first magnet means outside of the photosensitive body; and a discharge electrode firmly attached to a face of the second magnet means opposed to a photosensitive body surface. The discharge electrode has a predetermined distance from the photosensitive body surface.
In this structure, the magnets which have the same magnetic poles as those of the magnet provided inside of the photosensitive body or the magnetized base body are used with opposed configuration so that the repulsive force by the magnets can levitate the attached discharge electrode, thereby providing non-contact electrification. Moreover, this structure further comprises the discharge electrode firmly attached to a face of the second magnet means opposed to the photosensitive body surface, the discharge electrode having a predetermined distance from the photosensitive body surface, thereby avoiding the vibration of the discharge electrode.
The electrification apparatus according to the present invention is characterized in that the second magnet means comprises regulation means for regulating a magnetic pole direction of the second magnet means and a magnetic pole direction of the first magnet means so that these directions do not deviate from each other.
In this structure, the second magnet means provided outside of the photosensitive body has repulsion with the magnet inside of the photosensitive body or the magnetized base body. The opposite magnetic pole of the second magnet means is, however, drawn by the regulation means, thereby avoiding a rotation of the second magnet means. This allows the second magnet means to keep levitating with a constant distance.
The electrification apparatus according to the present invention is characterized in that the photosensitive body has an opening at least at its end. A support member is provided via the opening for fixedly supporting the first magnet means against a rotation of the photosensitive body.
In this structure, the support member fixedly supports the first magnet means on the photosensitive body and thus avoids the fluctuation of the levitating second magnet means to keep a constant distance between the magnets, thereby providing uniform discharge.
The electrification apparatus according to the present invention is characterized in that the first magnet means is positioned on a vertical line running through a rotation axis of the photosensitive body.
Since the first magnet means serves to levitate the second magnet means, the second magnet means is desirably provided on the vertical line. To do so, it is appropriate to provide the first magnet means at a position above the rotating photosensitive body.
The electrification apparatus according to the present invention is characterized in that the first magnet means is attached to elevation means.
In this structure, the levitation distance of the second magnet means is determined by the magnetic flux density (i.e., magnetic field intensity) of the first magnet means inside of the photosensitive body and the weight and magnetic flux density of the second magnet means outside of the photosensitive body. Constant intensity of electrification of an electrification unit requires minute adjustment of the distance between the photosensitive body surface and the second magnet means. This distance can be adjusted by vertically moving the first magnet means. Thus, the longitudinally movable installation of the first magnet means enables the intensity of electrification to be adjusted.
The electrification apparatus according to the present invention is characterized in that the image forming apparatus according to the present invention uses non-magnetic toner as a development agent for developing a latent image of the photosensitive body.
In this structure, in an electro-photography process, toner cleaned after the transfer step must not contact an electrification unit in subsequent processes. However, if insufficiently cleaned magnetic toner is used, this magnet-used electrification method cannot avoid a situation where magnets cause the magnetic toner to be attracted toward the electrification unit, resulting in a contaminated electrification unit which may cause a problem of uneven electrification. Thus, the use of the nonmagnetic toner can minimize uneven electrification.